Obviously you (most of you) have never used margin in real world.
For start, please make sure you actually understand how it works and how and why margin calls are calculated. Especially in illiquid and volatile markets, minimum margin requirement must be set properly or you will be fucked. If you do not fully understand wtf minimum margin requirement is, you will be fraked sooner rather than later and so will be your clients.
Next, you need to figure out how you can make sure your client wont run away with BTC they borrowed and shares/turds/bonds they have bought. In real world, share/bond/etc holder actually has no direct access to shares/bonds unless you hold bearer bonds (last ones will be extinct really soon) or have ordered physical delivery of your stock certificate (I did this once for fun and they looked beautiful
) Every trade you do in the real world, will be cleared through so called Clearing House. Even OTC trades. To be on a safe side, you need to copy that idea to your own business model. There is no need to waste your time and invent "something new".
Now, if you set up a "broker/clearing house" - you will keep your clients turds, bonds, shares etc - you can start thinking about margin trading. Your client can not run with your BTC and shares because you hold those for them.
1) Do not underestimate the importance of liquidity. If you can not sell your clients holdings and the account gets below minimum margin requirement - margin call - you will get hurt.
2) selling to cover a margin call for a client in previous example, can start a avalanche of margin calls in other accounts.
3) If you become flexible on margin calls (let it slide because ...) you will get hurt and so will your clients.
4) Better to set your margin calls lower than higher - You will get hurt bad, if the shit hits the fan and there is no liquidity (see 1 and 2).
5) do not "I hope it will go up so I recover blaa blaa" - you will get hurt.
6) do not get emotionally involved - you will get hurt. If calculations call for sell, you have to do it. (always see if you have matching buy orders)
7) do not get sucked in to 'fractional reserve banking' - No one wants to read (your) bullshit excuses how fiat/btc transfer will take "longer time" because you have some "mysterious technical problems" blaa blaa blaa
a.k.a you used your clients money to finance someones deals and now you have liquidity problems. Read up on "bank run"
Good luck and keep us posted.
BTW, if you like to ask questions or need to figure something out, send me a PM and I'll be happy to help you out.